Synthetic aperture radar (SAR) normally employs manned aircraft as the system platform. The military has developed for various other applications the unmanned airborne vehicle (UAV), but to date, SAR has been undeployable on smaller UAVs. These UAVs have a significant advantage, in that the small platform provides cost efficiencies as well as tactical advantages in the field, being easier to transport, carry, and deploy.
It would therefore be advantageous to utilize a smaller UAV in an SAR platform. To achieve this objective, it is essential to have an accurate determination of the UAV trajectory, because SAR requires UAV trajectory estimation accurate to the size of a range resolution cell. e.g. typically on the order of about 0.75 to about 2 meters, depending on the type of SAR system. The much smaller size of the UAV, however, relative to the manned aircraft approach, results in two major problems as a SAR platform:
U.S. Pat. No. 6,603,424, issued Aug. 5, 2003, T. J. Abatzoglou ('424 patent), and incorporated herein by reference, describes one approach for reducing errors in SAR signals. It observes that in generating the synthetic antenna, the signal processing equipment of an SAR operates on a basic assumption that the radar platform travels along a straight line trajectory at a constant speed. In practice, an aircraft carrying the radar antenna is subject to deviations from such non-accelerated flight. It is therefore necessary to provide compensation for these perturbations to straight-line motion. This motion compensation must be capable of detecting the deviation of the radar platform path from a true linear path.
Referring now to FIG. 1 (FIG. 1, '424 patent), an SAR system that is onboard an aircraft 10 maps a target region 12 by transmitting and receiving radar signals at various sampling points S1, . . . SN, along the flight path 14 of the aircraft. The '424 patent states that the SAR system may be positioned in the nose portion 15 of the aircraft. As the SAR system operates, errors can be introduced into the system that, if not compensated for, will corrupt the signal phase, possibly to the extent that the resulting degraded image is of no practical use. It continues that these errors may arise from a number of sources. e.g., errors in motion measurements, inaccurate acceleration estimates and atmospheric/ionospheric propagation effects, and that these errors can be arbitrary as with a wide-band random process.
Such errors are amplified in the present application, as due to weight constraints the available inertial navigational sensors are less accurate then the corresponding sensors for manned aircraft.
The small size of the UAV results in much larger perturbations of the UAV's programmed trajectory due to atmospheric turbulence and wind gusts.
To date, these problems have prohibited the use of such smaller UAVs in SAR applications.